Subaru Outback 6th gen: circumference matters

Two factory sizes, one height

225/65R17 measures 28.52 in and 225/60R18 measures 28.63 in. The difference is 0.11 in, or 0.40%, and the section width is identical at 8.86 in. One aspect-ratio step absorbs the extra inch of wheel almost exactly.

The sidewall is where they diverge: 5.76 in on the 17-inch fitment against 5.31 in on the 18.

The recorded steps are small on purpose

• 235/65R17 at 29.03 in — +0.51 in, 1.79% from 225/65R17, and +0.39 in of width.
• 245/65R17, 29.54 in tall and 0.79 in wider — +1.02 in, 3.59%.

Both stay inside the 3.6% band. On a vehicle with permanently engaged all-wheel drive, that restraint has a mechanical basis rather than an aesthetic one.

A centre differential distributes torque between axles and tolerates small differences in rolling circumference; a larger persistent mismatch is absorbed continuously by hardware designed to handle it only intermittently. Since a size change applies equally to all four corners, the front-to-rear mismatch is unaffected — but a mismatch between a new pair and a worn pair, or between four new tires and an original-size spare, is not.

That is why replacing tires in fours is standard advice on this platform and optional on many others, and it is the same arithmetic: tread wear alone moves rolling diameter by up to three quarters of an inch over a tire's life.

The 18-inch trim's option is to go down

The record lists 225/65R17 among the sizes owners of this platform fit, which looks odd until you read it from the 18-inch trim's perspective: moving to the 17-inch factory size is a −0.40% change, and it buys 0.44 in of sidewall and a cheaper wheel.

For a vehicle bought partly for unpaved roads, that swap is the most common modification made to it, and the arithmetic behind it is almost nothing — the speedometer moves by less than the difference between new and half-worn tires.

What 3.59% actually does

Fitting 245/65R17 from the 17-inch factory size:

• Diameter: +1.02 in
• Ride height at the axle: +0.51 in, half the diameter change
• Section width: +0.79 in
• Speedometer: an indicated 70 mph is 72.5
• Odometer: 100 indicated miles is 103.6 covered

Half an inch of additional ground clearance at the axle, for an inch of tire. On a crossover whose selling point is the clearance it already has, that is the honest scale of what the change delivers.

A belt has no steps for a taller tire to fall between

The ratio column is empty. At changes of this size the gearing consequence would be small whatever went in it, and the transmission fitted here makes it smaller still: a continuously variable drive answers a 3.59% taller tire by sitting at a marginally different ratio, not by shifting differently, so there is no step for the change to fall between.

Load index is the constraint nobody checks

A wagon of this kind is routinely loaded to capacity — roof rack, cargo area, four occupants and a trailer on the back — and the placard pressures were calculated against the original tire's load and inflation curve.

The two upgrade sizes are wider than the original, which does not by itself mean they carry more. Load capacity comes from the casing standard and the reference inflation, not from the width, and a wider tire in a lower load rating can carry less than the narrower one it replaced.

It takes two minutes with the load and speed index decoder to compare the service descriptions, and it is the check most often skipped on a size change that looks conservative.

What geometry does not decide

Room at the strut, room at the liner, room through the steering's full travel, and a load rating that still satisfies the placard. There is a fourth question on this car that the first three hide: with all four wheels driven through a centre differential, a mismatched set — three old tires and one new one, or a factory-size spare after an upgrade — is a difference the driveline has to absorb continuously rather than a nuisance it can ignore.